1. Field of the Invention
This invention relates to a reflection type liquid crystal display device and also to a method of manufacturing the reflection type liquid crystal display.
2. Description of the Related Art
A liquid crystal display device (hereinafter referred simply as LCD) is extensively utilized in word processors, personal computers, projection type TV's or miniature TV's.
Recently, a reflection type liquid crystal display device which requires no back-light has been attracting much attention. Namely, since this reflection type liquid crystal display device can be used as a display device in office automation (OA) apparatuses without necessitating any back-light, it consumes a lesser amount of electric power, and therefore is suited for use in portable equipment. However, since this reflection type liquid crystal display device makes use of the external light, it can not be satisfactorily used unless the reflectance of the LCD is not sufficiently high.
The reflection type liquid crystal display device can be classified according to the degree of reflectance thereof into three groups, i.e. a display mode where a couple of polarizing plates are used; a display mode where a single polarizing plate is used; and a display mode where no polarizing plate is used at all.
FIG. 1 shows a TN type LCD as an example of a display mode where a couple of polarizing plates are used. According to this TN type LCD, each of an upper plate 1 and a lower plate 2 are provided with a transparent electrode 3 and a transparent electrode 4, respectively, and a liquid crystal composition layer 5 is interposed between the upper plate 1 and the lower plate 2. The respective outer surface of substrates 1 and 2 is attached with a polarizing plate 6a and 6b. Further, on the outer surface of the polarizing plate 6b attached to the lower substrate 2 is attached a diffusion reflecting plate 7. The optical path is formed in this TN type LCD such that light passes through the polarizing plate four times and the substrate four times. The light transmittance through the polarizing plate in a single pass is theoretically 50% or less, and actually a little more than 40%. Since the light would be absorbed by the other polarizing plate and the substrates, the reflectance may be ultimately greatly reduced.
FIG. 2 shows a single polarizing plate mode ECB type LCD having only one polarizing plate 6 as an example of a display mode where a single polarizing plate is used. In contrast to the TN type LCD, the optical path is formed in this ECB type LCD such that light passes through the polarizing plate only two times and the substrate only two times. In FIG. 2 as well as in the following figures, the same portions with those of the previous figures will be identified by the same reference numerals. The light transmittance through the polarizing plate is theoretically 50% or less in a single pass and actually a little more than 40% in this example as in the previous example. However, since the light absorption is reduced by an amount corresponding to passing two times through the polarizing plate and two times through the substrate, the resultant light reflectance obtained would be a little higher than that in the case of the TN type LCD.
As examples of the display mode where no polarizing plate is used at all, a high polymer PC-GH type LCD having a guest-host liquid crystal composition layer 5a is shown in FIG. 3, a GH-HOMO type LCD having a guest-host liquid crystal composition layer 5b is shown in FIG. 4, and a double-layered GH-HOMO type LCD having two guest-host liquid crystal composition layers 5b with a common substrate 8 being interposed therebetween is shown in FIG. 5.
Since the polarizing plate is not employed at all in any of the systems shown in FIGS. 3 to 5, it is possible in these systems to brighten a display by an amount corresponding to one pass of light through the polarizing plate whose light transmittance is theoretically 50% or less and actually a little more than 40%. Moreover, if a reflecting plate is attached on the inner side of the cell as in the case of the single polarizing plate mode ECB type LCD mentioned above, it is possible to reduce the light absorption by an amount corresponding to two passes of light through the substrate, thus prominently improving the light reflectance as compared with a display mode employing the polarizing plate.
The reflection type LCD shown in FIG. 6 is a modification of the GH-HOMO type LCD shown in FIG. 4 and a quarter wavelength plate 9 is interposed between the reflecting plate 7 and the liquid crystal cell. In this reflection type LCD, incident light, after passing through the liquid crystal cell, then passes through the quarter wavelength plate 9 and is reflected by the reflecting plate 7, thus passing again through the quarter wavelength plate 9, the phase thereof being shifted by one half of its wavelength, and then interring again into the liquid crystal cell.
According to this reflection type LCD, it is possible with a structure having only one liquid crystal layer and one layer of liquid crystal cell to perform light control in the same manner as in the double-layered GH-HOMO type LCD shown in FIG. 5.
According to these reflecting type LCDs, a display is generally effected by applying a voltage to a liquid crystal layer through electrodes, by passing electric current to a liquid crystal layer, or by applying a magnetic field. Further, since electrodes are employed in these reflecting type LCDs, an insulating region (a region where an electrode is not disposed) is certainly required. In particular, in the case of a display where various patterns such as characters, drawings or images are to be displayed, the electrodes are arranged in a matrix pattern. When electrodes are arranged in a matrix pattern, an interconnecting wiring is also generally required to be provided in addition to the electrodes provided for applying voltage to the liquid crystal layer. Even if electrodes are arranged, not in a matrix pattern, but other some complex patterns (for example, 7-segment display used in a miniature electric calculator or watch), an interconnecting wiring is also required. Thus, the LCD is formed of two regions, i.e. an electrode region provided for applying voltage to the liquid crystal layer and an insulation region, or three regions in some cases, i.e. an electrode region provided for applying voltage to the liquid crystal layer, an insulation region and a wiring region.
In the following description of this specification, a region of light reflecting layer whose liquid crystal can be modulated in response to the voltage from electrodes provided for applying voltage to the liquid crystal layer will be referred to as a modulation region, and a region other than this modulation region will be referred to as a non-modulation region. Likewise, an insulating region in the non-modulation region will be referred to as a space region, and a region where an interconnecting wiring is disposed in the non-modulation region will be referred to as a wiring region.
By the way, a light-shielding layer is often formed in the non-modulation region in a transmission type LCD. This light-shielding layer, when it is applied in a matrix display, is generally called a black matrix (BM). The provision of this light-shielding layer is intended to improve the contrast characteristic of display. Meanwhile, the mode of the LCD can be classified according to the system of controlling the display thereof (irrespective of whether it is a transmittance type or a reflecting type) into two groups, i.e. a normally-white mode (hereinafter referred to as NW mode) where a white state is obtained without being impressed with voltage, and a normally-black mode (hereinafter referred to as NB mode) where a white state is obtained only when a voltage is impressed.
In the case of NW mode, the non-modulation region is almost always kept in a white state irrespective of the condition of the modulation region. Therefore, when a light-shielding layer is not provided, the brightness of the black state as a whole (the modulation and non-modulation regions) is higher than when the shielding layer is provided. Accordingly, the contrast ratio (the brightness of the white state/the brightness of the black state) can be improved by providing a light-shielding layer. On the other hand, in the case of NB mode, the non-modulation region is almost always kept in a black state irrespective of the condition of the modulation region. Therefore, the brightness of the black state as a whole (the modulation and non-modulation regions) is darker as compared with the NW mode. However, the black state of the generally known NB mode is not sufficiently dark. Because of this, the shielding layer is provided thus rendering the brightness of the non-modulation region to zero to a full extent, thereby achieving a high contrast.
However, the provision of the light-shielding layer causes the lowering of the display brightness (luminosity) as a whole. This is a disadvantage of the light-shielding layer. However, in the case of the transmittance type LCD, a back light is utilized so that it is possible to maintain a sufficient degree of the display brightness (luminosity) by improving the brightness of this back light. Therefore, a light-shielding layer is frequently employed for attaining a sufficient degree of contrast.
By contrast, in the case of the reflecting type LCD, light that can be utilized is limited to the external light, so that it is impossible for the display side to control the incident light intensity. Accordingly, if the light-shielding layer is applied to a reflecting type LCD, the display brightness may be extremely lowered. Therefore, the light-shielding layer is not generally applied to the reflecting type LCD (however, since TFT acting as a driving element is influenced by light, a shielding layer is occasionally formed at a required portion of a non-modulation region). To begin with, the reflecting type LCD rely on the external light for its light source, and is incapable of controlling an incident light, so that the most important characteristic demanded is the luminosity (brightness). Because of this reason, there is no light-shielding layer on the non-modulation region of the conventional ordinary reflecting type LCD.
However, even if the light-shielding layer is omitted, the conventional reflecting type LCD still fails to achieve a sufficient degree of display brightness. In the case of NW mode, the non-modulation region is almost always kept in a white state, so that it is possible to attain some degree of display brightness as a whole. However, in the case of the reflecting LCD, since a polarizing plate or dye is employed for obtaining a black state, it can not actually avoid the light absorption to some extent even if it is in a white state. As a result, it is impossible to obtain a sufficient degree of brightness. On the other hand, in the case of the NB mode, since the non-modulation region is almost always kept in a black state, the display brightness as a whole is naturally low.
As explained above, when the reflecting type liquid crystal display device is employed as a display, only low reflectance could be obtained even in the non-modulation region. In particular, this tendency is prominent in the case of normally-black mode, so that the display brightness as a whole is very low.